Continuous wave (CW) spectrophotometers have been widely used to determine in vivo concentration of an optically absorbing pigment (e.g., hemoglobin, oxyhemoglobin) in biological tissue. The CW spectrophotometers, for example, in pulse oximetry introduce light into a finger or the ear lobe to measure the light attenuation and then evaluate the concentration based on the Beer Lambert equation or modified Beer Lambert absorbance equation. The Beer Lambert equation (1) describes the relationship between the concentration of an absorbent constituent (C), the extinction coefficient (.epsilon.), the photon migration pathlength &lt;L&gt;, and the attenuated light intensity (I/I.sub.o). ##EQU1## However, direct application of the Beer Lambert equation poses several problems. Since the tissue structure and physiology vary significantly, the optical pathlength of migrating photons also varies significantly and can not be simply determined from geometrical position of a source and detector. In addition, the photon migration pathlength itself is a function of the relative concentration of absorbing constituents. As a result, the pathlength through an organ with high blood hemoglobin concentration, for example, will be different from the same with a low blood hemoglobin concentration. Furthermore, the pathlength is frequently dependent upon the wavelength of the light since the absorption coefficient of many tissue constituents is wavelength dependent. One solution to this problem is to determine .epsilon., C, and &lt;L&gt; at the same time, but this is not possible with the pulse oximeters known previously.
Furthermore, for quantitative measurement of tissue of a small volume (e.g., a finger) photon escape introduces a significant error since the photons escaped from the tissue are counted as absorbed. Other errors may occur due to irregular coupling of light to the examined tissue or varying relative geometry of the input and detection ports.
The time resolved (TRS-pulse) and phase modulation (PMS) spectrophotometers can measure the average pathlength of migrating photons directly, but the proper quantitation of the time resolved or frequency resolved spectra can be performed only when the spectra are collected at a relatively large source-detector separation. This separation is difficult to achieve for a small volume of tissue such as the earlobe, a finger or a biopsy tissue.
Therefore, there is a need for an optical coupler used with a spectrophotometric system and method that quantitatively examines a relatively small volume of biological tissue.